When a telecommunications terminal is used in speakerphone mode, acoustic echo or “feedback” between the loudspeaker and the microphone of the telecommunications terminal is likely to occur. When acoustic echo occurs, sound produced by the loudspeaker is sensed by the microphone and retransmitted back to its original source, causing undesirable artifacts, such as echo, to be heard.
In accordance with acoustic echo canceling, a digital-adaptive filter models the acoustic coupling between loudspeakers and the microphone of a telephone receiver (i.e., the “acoustic signature”) in the room. The audio signal that feeds the local loudspeaker is filtered using the acoustic signature stored in the adaptive filter model. This filtered signal is then subtracted from the microphone signal before the signal is transmitted to the far end, resulting in an improved signal under some conditions. However, acoustic echo canceling has some limitations. First, the adaptive filter might take several seconds to adapt to changes in the acoustic signature of the room. Second, adaptation can only occur when the far-end party is talking but the local parties are not. As a result, if changes to the room's acoustic signature occur during a conversation—for example, the microphone moves or a local party's body moves with respect to the local microphones—then the far-end party will hear the artifacts of inadequate cancellation until the digital filter can re-adapt.
Acoustic echo suppression is used to supplement the acoustic echo canceling when it is determined that the echo canceling is not sufficient to prevent acoustic echo. Acoustic echo suppression involves the insertion of mild attenuation in the microphone signal in order to prevent remaining echo signal from being transmitted. When echo suppression is used, the users of the telecommunications terminal often experience a “half-duplex” effect, where the speech of the speakerphone user is clipped or suppressed as a result of the echo suppression. When such clipping or suppression occurs, its cause is not readily apparent to the user of the telecommunications terminal, leading the user to become frustrated as he or she does not know when his or her speech is heard by the far-end participant nor does he or she know how to react in order to stop the voice clipping.